1. Field of the Invention
The present invention relates to an oxide dispersion-strengthened alloy, and more specifically, to an oxide dispersion-strengthened nickel alloy suitable as a material for high temperature equipment of a gas turbine such as a combustor and the like of the gas turbine.
2. Description of the Related Art
There is a tendency that combustion gas temperature of an industrial gas turbine used for power generation and the like is increased to improve thermal efficiency. For this purpose, a component such as a combustor, bucket and nozzle guide vane of a gas turbine and the like is exposed to a temperature higher than a conventional temperature. On the other, as the temperature of a combustion gas is increased, an increased amount of a nitrogen oxide (hereinafter, abbreviated as NOx) is generated in combustion. Thus, the amount of the generated NOx must be reduced. The most efficient method of reducing an amount of generated NOx is to take out part of compressed air which is used conventionally for component cooling and use the part so that the fuel concentration in a gas mixture composed of the compressed air and fuel is rarefied prior to combustion. Consequently, a material used for a gas turbine which is in contact with a combustion gas having an increased temperature is required to have greatly improved resistance to temperature from the two factors that first a combustion gas has an increased temperature by itself and secondly an amount of cooling air is reduced. Many types of alloys with strength increased at high temperature mainly composed of a Ni alloy were developed and proposed in the past by the improvement of an alloy composition and a manufacturing process and used to a bucket and nozzle guide vane which were made by casting in the equipment constituting a gas turbine. Since a .gamma.' phase as a strengthening phase of the Ni alloy is decomposed and vanishes in a high temperature region at 900.degree. C. or higher, however, it is contemplated to be difficult to greatly improve the resistance to temperature of the Ni alloy which is used now. Further, since a material used for the liner and transition piece of a combustor is required to have a hot workability besides high temperature strength so that the material can be rolled to a sheet, any alloy having greatly improved strength as compared with that of existing forging alloys such as Hastelloy X of the Ni alloy, HA188 of a Co alloy and the like is not yet put into practical use.
On the other hand, there are developed oxide dispersion-strengthened alloys as an alloy having resistance to temperature higher than that of conventional forging alloys and casting alloys which is prepared by finely dispersing oxide particles in matrix by a mechanical alloying method. These alloys are developed by an idea that the movement of dislocation generated in the matrix is prevented up to a high temperature region near to the melting point of the alloys by stable oxide particles. A method of manufacturing the oxide dispersion-strengthened alloy is disclosed in Japanese Patent Unexamined Publication No. 47-42507 (1972) and the like by which pure metal, alloy powder and fine powder of oxide such as Y.sub.2 O.sub.3 and the like as materials are mechanically mixed in a high energy ball mill and then solidified by sintering and further hot worked and heat-treated to grow crystal grains coarse for high temperature use.
This type of the oxide dispersion-strengthened alloys are roughly classified into Ni alloys in which an oxide is mixed with a .gamma.' phase to increase strength and alloys strengthened only by an oxide. Since the former alloys strengthened by the mixture of the oxide with the .gamma.' phase are strong at a temperature less than 900.degree. C., the application of them to the bucket of a gas turbine is considered. Since it is very difficult to hot forge and hot roll the alloys, however, the application of the alloys to the portion such as the liner of a combustor, and the like where a hot working property is required is difficult. Further, since the .gamma.' phase is decomposed at a temperature higher than 900.degree. C., the application of the alloys to the portion such as a first stage nozzle guide vane which is expected to be used at a temperature exceeding 900.degree. C. is difficult. On the other hand, since the latter alloys which are strengthened only by the oxide phase are excellent in a hot rolling property and a structure stability at a high temperature higher than 900.degree. C., Ni alloys such as MA754 and the like are proposed as a material for the nozzle guide vane of an aircraft and a Fe alloy referred to as MA956 is proposed as a material for a combustor by inco Co. Ltd. in U.S.A., respectively.
However, when it is taken into consideration that the combustion temperature of an industrial gas turbine is increased in the future and an amount of generated NOx is increased accordingly, it is expected that an amount of cooling air to be supplied to high temperature equipment such as a first stage nozzle guide vane, combustor and the like must be more reduced than a present amount. In particular, it is also expected that the liner and transition piece of the combustor are used only by convention cooling on the outside wall thereof without film cooling on the inside wall thereof in an extreme case. In such a case, there is a possibility that a material is used at a high temperature reaching 900.degree. or higher and in some cases at a temperature exceeding 1000.degree. C. even if a heat prevention coating is applied onto the surface of the material. At such a high temperature, the existing oxide dispersion-strengthened Ni alloys do not have a sufficient creep rupture strength and thermal fatigue characteristics which are necessary as a structure material. All of these existing oxide dispersion-strengthened Ni alloys contain aluminum in the alloy composition thereof to improve resistance to oxidation at high temperature and to absorb oxygen contained in material powder. Since the oxide dispersion-strengthened alloys use fine metal powder as a material, a content of oxygen which is excessive as compared with that of casting alloys cannot be avoided. The conventional oxide dispersion-strengthened alloys absorb oxygen by forming Al.sub.2 O.sub.3 by mainly adding aluminum. According to a recent study, it becomes apparent that Al.sub.2 O.sub.3 reacts with Y.sub.2 O.sub.3 as a strengthening phase in a manufacturing process and an Y--Al composite oxide is produced. When Al.sub.2 O.sub.3 is mixed with Y.sub.2 O.sub.3 as described above, a resistance force against the movement of dislocation is lowered by coarsened oxide particles and accordingly the creep rupture strength and thermal fatigue characteristics of the alloys are also lowered. Aluminum contained in the oxide dispersion-strengthened Ni alloys is very active to Y.sub.2 O.sub.3 and even if the aluminum content of a material corresponding to MA754 is only 0.3 wt %, almost all the Y.sub.2 O.sub.3 contained in the alloy is mixed with the aluminum.
Taking the above problems into consideration, an object of the present invention is to provide an oxide dispersion-strengthened Ni alloy having improved high temperature creep characteristics and heat resistant fatigue characteristics while maintaining a hot rolling property and a structure stability at high temperature and to provide high temperature equipment such as the nozzle guide vane, combustor and the like of an industrial gas turbine which can improve the efficiency of the gas turbine without increasing an amount of generated NOx by using the alloy as a structural member thereof.